INTERNAL ENVIRONMENT, MICROCLIMATE AND RESOURCES - Soil Ecology - page 107

Biological fixation by higher plants and some micro-organisms provide inputs of
atmospheric carbon to ecosystems. Nitrogen, despite its predominance in the atmosphere,
is largely unavailable to higher plants except through fixation by prokaryotes. These
organisms may be symbiotic, associative or free living, although species of intermediate
habits also occur. Of great agricultural and silvicultural importance are the bacteria of
the genera Rhizobium, Bradyrhizobium and Azorhizobium that form symbioses with
a wide range of tree, shrub, pasture and cropped legumes; however, not all legumes
form such associations. A number of other groups also fix nitrogen with varying degrees
of effectiveness. Actinobacteria of the genus Frankia form nitrogen-fixing symbioses with
a range of species (mainly trees) belonging to several families (Quispel et al., 1993).
Other symbiotic systems include the ancient associations of the lichens and the cycads
with cyanobacteria (Grobbelaar, 1993). Sugarcane and some other tropical grasses,
have been shown to fix nitrogen in association with bacteria of the genus Azospirillum
(Sprent and Sprent, 1990). The cyanobacterium Anabaena azollae forms a symbiosis
with aquatic ferns of the genus Azolla which is important in the nitrogen economy of rice
crops in some areas. A range of free-living bacteria, both aerobes and such anaerobes as
Azotobacter spp. and Clostridium spp., are widespread in soils and fix lesser but still
significant quantities of nitrogen.
Lastly, in many agricultural situations, fertilisers are frequently the major source of
a number of plant nutrient elements for crop plants and the simplified ecosystems of
which they form the autotrophic base.

3.1.4 MECHANISMS OF STORAGE AND RELEASE

3.1.4.1 Nutrient stocks (standing crops)

As indicated above, nutrient elements are derived from several different sources and
possess a wide range of mobilities and turnover times within the soil. The stock of a
particular element at a given site is defined as the total mass per unit area of that element
lying within a defined soil depth range, often the root zone. It may also be defined in
terms of the masses of particular fractions (often chemically defined) of such elements.
Such fractionations frequently aim to define a 'plant-available' fraction and this may
have restricted value in the context of crop growth within limited ranges of soil and
environmental conditions. Despite the above, fractionations based on functionally-realistic
criteria have proved very insightful, as discussed above in the context of phosphorus
fractionation (Cross and Schlesinger, 1995). As discussed below, fractionations on
various bases have also proved useful in studies of soil organic matter (see Figure I.13)
and in estimation of the nutrient element composition of the microbial biomass.
Nonetheless, the magnitudes and depth distributions of nutrient stocks in soils provide
an estimate of the total amounts of particular elements contained within a given soil
mass, or volume. Such estimates may reflect soil age, the pedological processes occurring
within them, the nature of their vegetative cover and environmental conditions.
Nutrient elements are not distributed evenly within soils: through biological and
pedological processes, they become concentrated at particular locations and remain
substantially lower throughout the remaining soil volume. Because of ecosystem recycling